The present invention relates to a method and apparatus for processing a signal. More specifically, the present invention relates to processing a signal to detect the occurrence of an external event.
Detection devices determine when a particular event occurs. Examples of detection devices include beam sensors such as those used in automatic doors. The beam sensor determines when a person has approached the door by detecting when the beam has been reflected or broken. Once the person is detected by the sensor, the door is automatically opened for the user. Other examples include label sensors. Label sensors detect labels as they pass by the sensor on a web. The detection of the label allows proper high-speed counting of labels as well as proper removal of the labels from the web.
Conventionally, the electronics of such detection devices require a user to make threshold adjustments. This adjustment is necessary due to variations in the sensing conditions. In the label sensor example, web flutter and label inconsistencies create noise in the signal. A threshold must be properly chosen to distinguish a rising or falling label edge signal from the noise. Because the web flutter and label inconsistencies change from one web to the next, the threshold must be adjusted by the user each time a new web is analyzed to produce accurate results.
Also, the electronics of detection devices require a user to make offset adjustments. An offset adjustment maintains a baseline amplified signal at a desired midway point between two sensing thresholds, one for a falling signal and one for a rising signal. These rising and falling signals represent transitions created by an external event. For instance, when a leading label edge is sensed the signal rises to a level indicating the label is present. When a trailing label edge is sensed the signal falls to a level indicating the label is not present. The signal must rise beyond a rising threshold to indicate that a leading label edge has been sensed, and the signal must fall below a falling threshold to indicate a trailing label edge has been sensed.
Amplifiers are used in the detection device to scale the detection signal to useful levels, and these amplifiers are susceptible to fluctuations in performance and calibration due to humidity, ambient temperature, and other factors. The fluctuations affect the level of the baseline signal which then affects the reliability of the detection since the baseline signal may stray too far from a threshold for a rise or fall to be properly detected. A user must continuously monitor and adjust the baseline signal to ensure that it remains midway between the two threshold levels.
Furthermore, external events may occur randomly with no consistency, and detection of these event is often done in a noisy environment. Determining the occurrence of such events is difficult because the noise can produce false detections. Furthermore, accurate detection of randomly occurring events is difficult because resulting detection signals may range from nearly DC to very high frequencies, thus rendering differentiation schemes optimized for a given frequency range virtually inoperable.
Detection sensors would be more effective if no user adjustments were necessary due to variations in the environment including noise, ambient conditions, and the disparity of event frequency. Eliminating the user threshold adjustment and/or user offset adjustment improves the ease of use of the sensor as well as the reliability of the results it produces. Similarly, providing a system that can differentiate between detection signals spanning from DC to a much higher frequency further improves the ease of use and reliability of the sensor.
The present invention addresses these problems by removing the need of the user to adjust the system to optimize the threshold level and the offset of the amplifier based on the sensing conditions and the particular characteristics of the event being detected. Furthermore, the present invention addresses the problem of accurately detecting events with widely varying durations in noisy environments.
The invention is embodied in apparatuses and methods for detecting a change in a signal having a level that fluctuates in response to the external event. The apparatus includes an adjustable amplifier module for generating an amplified signal and having a signal input and an offset input. The signal input of the amplifier module is electrically connected to the signal to be detected. An offset adjusting module may be included to continually adjust the offset voltage of the adjustable amplifier module by generating an offset signal based upon the amplified signal to maintain an observed median value of the observed range for the amplified signals between a minimum and a maximum observed value. The offset signal is coupled to the offset input of the adjustable amplifier module.
The apparatus may also include a cascaded difference filter module for receiving the amplified signal to detect an occurrence of the external event by detecting when the amplified signal changes to a value greater than a threshold value. The cascaded difference filter quantifies the change in signal over multiple time intervals. A threshold adjusting module may also be included to continually adjust the threshold value based upon the amplified signal to maintain the threshold value at a desired percentage of a maximum range of observed amplified signals.